To meet the demand for wireless data traffic having increased since deployment of 4G communication systems, efforts have been made to develop an improved 5G or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post LTE System’. The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems. In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like. In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
The Internet, which is a human centered connectivity network where humans generate and consume information, is now evolving to the Internet of Things (IoT) where distributed entities, such as things, exchange and process information without human intervention. The Internet of Everything (IoE), which is a combination of the IoT technology and the Big Data processing technology through connection with a cloud server, has emerged. As technology elements, such as “sensing technology”, “wired/wireless communication and network infrastructure”, “service interface technology”, and “Security technology” have been demanded for IoT implementation, a sensor network, a Machine-to-Machine (M2M) communication, Machine Type Communication (MTC), and so forth have been recently researched. Such an IoT environment may provide intelligent Internet technology services that create a new value to human life by collecting and analyzing data generated among connected things. IoT may be applied to a variety of fields including smart home, smart building, smart city, smart car or connected cars, smart grid, health care, smart appliances and advanced medical services through convergence and combination between existing Information Technology (IT) and various industrial applications.
In line with this, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as a sensor network, Machine Type Communication (MTC), and Machine-to-Machine (M2M) communication may be implemented by beamforming, MIMO, and array antennas. Application of a cloud Radio Access Network (RAN) as the above-described Big Data processing technology may also be considered to be as an example of convergence between the 5G technology and the IoT technology.
Mobile communication systems have been developed into high speed and high quality wireless packet data communication systems to provide data services and multimedia services as well as voice services that had been provided as initial services. Recently, various mobile communication standards, such as high speed downlink packet access (HSDPA), high speed uplink packet access (HSUPA), long term evolution (LTE), and LTE advanced (LTE-A) of the 3rd Generation Partnership Project (3GPP), high rate packet data (HRPD) of 3GPP2, or the Institute of Electrical and Electronics Engineers (IEEE) 802.16, have been developed to support high speed and high quality wireless packet data transmission services. More particularly, an LTE system, which has been developed to effectively support high speed wireless packet data transmission, maximizes the capacity of a wireless system using various wireless connection technologies. An LTE-A system is an advanced system of the LTE system, and has a more enhanced data transmission ability compared with the LTE system.
The LTE generally means base stations and terminals corresponding to Release 8 or 9 in the 3GPP standardization organization, and the LTE-A denotes base stations and terminals corresponding to Release 10 in the 3GPP standardization organization. After standardization of the LTE-A system, the 3GPP standardization organization has proceeded with standardization of subsequent Releases, which have enhanced performance, based on the LTE-A system.
The current third and fourth generation wireless packet data communication systems, such as HSDPA, HSUPA, HRPD, LTE/LTE-A, or the like, utilize an adaptive modulation and coding (AMC) method and a channel sensitive scheduling method in order to improve transmission efficiency.
By using the AMC method, a transmitter may adjust the amount of data to be transmitted according to a channel state. For example, in a bad channel state, the amount of data to be transmitted is reduced to thereby adjust the receiving error probability to a desired degree, whereas, in a good channel state, the amount of data to be transmitted is increased to thereby adjust the receiving error probability to a desired degree and effectively transmit a lot of information. In using the channel sensitive scheduling resource management method, since a transmitter selectively provides services to user terminals that are in good channel states among a multitude of user terminals, the system capacity may increase compared to a method by which a channel is allotted to a single user for services. Such an increase in the capacity of the system is called multi-user diversity. In other words, according to the AMC method and the channel sensitive scheduling method, partial channel state information is fed back from a receiver and a proper modulating and encoding method is applied at the time when it is determined to be most efficient.
The AMC method may include a function of determining the number or the rank of spatial layers of transmission signals when it is used together with a multiple input multiple output (MIMO) transmission method. In this case, the AMC method may determine an optimum data rate by considering the number of layers by which data is transmitted using the MIMO method as well as by considering an encoding rate and a modulating method.
The MIMO system, which transmits wireless signals using a plurality of transmission antennas, may be divided into a single-user (SU)-MIMO system in which transmission is made to a single user terminal, and a multi-user (MU)-MIMO system in which transmission is made to a plurality of user terminals using the same time and frequency resources. In the case of the SU-MIMO system, a plurality of transmission antennas transmits wireless signals to a single receiver through a plurality of spatial layers. At this time, the receiver should have a plurality of reception antennas for supporting the plurality of spatial layers. On the contrary, in the case of the MU-MIMO system, a plurality of transmission antennas transmits wireless signals to a plurality of receivers through a plurality of spatial layers. The MU-MIMO system does not require a receiver that has a plurality of reception antennas, compared to the SU-MIMO system. However, since the MU-MIMO system transmits wireless signals to a plurality of receivers using the same frequency and time resources, interference may occur in wireless signals for different receivers.
Meanwhile, recently, conversion from code division multiple access (CDMA), which had been used in the second and the third mobile communication systems into orthogonal frequency division multiple access (OFDMA) used in the next generation mobile communication system has been widely researched. 3GPP and 3GPP2 began to proceed with standardization about an evolution system that uses OFDMA. It is known that the OFDMA system can increase the capacity compared to the CDMA system. One of the reasons why the OFDMA system comes with an increase in the capacity is frequency domain scheduling on the frequency axis. A capacity gain can be obtained through the channel sensitive scheduling method according to characteristics in which the channel varies with time. Likewise, further capacity gain can be obtained using characteristics in which the channel varies with frequency.
FIG. 1 illustrates time and frequency resources in LTE/LTE-A systems according to the related art.
Referring to FIG. 1, wireless resources transmitted from a base station, i.e., an evolved Node B (eNB), to a user terminal may be divided by a unit of resource blocks (RB) on the frequency axis and by a unit of subframes on the time axis, respectively. In general, the RB is comprised of twelve subcarriers and has a bandwidth of 180 kHz in the LTE/LTE-A systems. On the contrary, the subframe is comprised of fourteen OFDM symbol sections and has a time section of 1 msec in the LTE/LTE-A systems. In scheduling, the LTE/LTE-A systems may allot resources by a subframe unit on the time axis and by an RB unit on the frequency axis.
FIG. 2 illustrates a minimum wireless resource of one subframe and one RB for scheduling through downlink in LTE/LTE-A systems according to the related art.
Referring to FIG. 2, the wireless resource is comprised of one subframe on the time axis and one RB on the frequency axis, respectively. The wireless resource includes twelve subcarriers in a frequency area and fourteen OFDM symbols in a time area to thereby have 168 natural frequency and time regions in total. Each natural frequency and time region shown in FIG. 2 is referred to as a resource element (RE) in the LTE/LTE-A systems. In addition, one subframe is comprised of two slots that each have seven OFDM symbols.
The wireless resources of FIG. 2 may transmit a plurality of different signals as follows.                1. Cell specific reference signal (CRS): A reference signal that is transmitted to all of the user terminals included in one cell,        2. Demodulation reference signal (DMRS): A reference signal that is transmitted to a specific user terminal,        3. Physical downlink shared channel (PDSCH): A data channel that is transmitted through downlink using an RE by which a reference signal is not transmitted in the data region of FIG. 2, to allow a base station to transmit a traffic signal to a user terminal,        4. Channel status information reference signal (CSI-RS): A reference signal transmitted to user terminals included in one cell and used for measuring channel status. A plurality of CSI-RSs may be transmitted in one cell, and        5. Other control channels (physical hybrid automatic repeat-request (HARQ) indicator channel (PHICH), physical control format indicator channel (PCFICH), or physical downlink control channel (PDCCH)): These provide control information required for the user terminal to receive the PDSCH, or transmit acknowledge/negative-acknowledge (ACK/NACK) for managing HARQ for data transmission through uplink.        
In addition to the signals above, a muting function may be configured so that the CSI-RSs transmitted from other base stations can be received by user terminals in a corresponding cell without interference in the LTE-A system. The muting function may be applied in the region where the CSI-RS can be transmitted, and generally, the user terminal receives a traffic signal while skipping the corresponding wireless resource to which the muting function is applied. In the LTE-A system, the muting may be called a zero-power CSI-RS as well. This is because the muting is applied to the region of the CSI-RS and transmission power is not transmitted due to the characteristics of the muting.
As shown in FIG. 2, the CSI-RS may be transmitted using some of the regions denoted by A, B, C, D, E, F, G, H, I, and J according to the number of antennas that transmit the CSI-RS. In addition, the muting may be applied to some of the regions denoted by A, B, C, D, E, F, G, H, I, and J. More particularly, the CSI-RSs may be transmitted by two, four, or eight REs according to the number of antenna ports that transmit the same. For example, in the case of two antenna ports, the CSI-RS may be transmitted using half a specific pattern, and in the case of four antenna ports, the CSI-RS may be transmitted using one specific pattern as a whole. Furthermore, in the case of eight antenna ports, the CSI-RS may be transmitted using two patterns. Contrarily, the muting is always made by one pattern unit. For example, although the muting may be applied to a plurality of patterns, when the muting region does not overlap the region of the CSI-RS, the muting cannot be applied to a part of one pattern. However, when the CSI-RS region does not overlap the muting region, the muting may be applied to a part of one pattern.
A reference signal should be transmitted in order to measure a downlink channel state in a cellular system. In the case of the LTE-A system of 3GPP, the user terminal measures a channel state between the base station and the user terminal using the CRS or the CSI-RS that are transmitted by the base station. The channel state should consider several factors including the amount of interference in downlink. The amount of interference in downlink includes interference signals and thermal noise generated by antennas included in a nearby base station, and it is important for the terminal to determine a channel state of downlink.
For example, when a signal is transmitted from a base station having a single transmission antenna to a user terminal having a single reception antenna, the user terminal should determine a signal to noise plus interference ratio (SNIR) by determining energy per symbol, which can be received through downlink, and the amount of interference that is to be simultaneously received in the section where the corresponding symbol is received, from a reference signal received from the base station. The SNIR is a value obtained by dividing power of a reception signal by the intensity of interference and noise signals. In general, the higher the SNIR is, the better reception performance and the higher data transmission speed can be obtained. The determined SNIR, a value corresponding thereto, or the maximum data transmission speed supported by the corresponding SNIR is notified to the base station so that the base station determines the data transmission speed from the base station to the user terminal through downlink.
In the case of a general mobile communication system, base station equipment is disposed at the intermediate region of each cell, and the corresponding base station equipment performs mobile communication with the user terminals using one or more antennas located in a limited place. The mobile communication system in which antennas included in one cell are disposed at the same place is called a centralized antenna system (CAS). On the contrary, the mobile communication system in which antennas (e.g., remote radio head (RRH)) included in one cell are disposed at distributed places is called a distributed antenna system (DAS).
FIG. 3 illustrates a deployment of antennas in a general distributed antenna system according to the related art.
Referring to FIG. 3, a distributed antenna system having two cells 300 and 310 is illustrated. The cell 300 has one high power antenna 320 and four low power antennas 340. The high power antenna 320 may provide services to the entire cell area. On the contrary, the low power antennas 340 may provide a high data speed-based service to limited user terminals in limited cell areas. In addition, the low power antennas 340 and the high power antenna 320 may be connected with a central controller as denoted by a reference numeral 330 to be operated according to scheduling and wireless resource allotment by the central controller. In the distributed antenna system, one or more antennas may be disposed at a single antenna region that is geographically separated. The antenna or the antennas which are disposed at the same region in the distributed antenna system is referred to as an antenna group (RRH group).
As shown in FIG. 3, in the distributed antenna system, the user terminal receives signals from a single antenna group that is geographically separated, whereas signals transmitted from the other antenna groups act as interference.
FIG. 4 illustrates generation of interference when each one of antenna groups transmits signals to different user terminals in a distributed antenna system according to the related art.
Referring to FIG. 4, a first user equipment (UE1) 400 receives a traffic signal from an antenna group 410. On the contrary, a second user equipment (UE2) 420, a third user equipment (UE3) 440, and a fourth user equipment (UE4) 460 receive traffic signals from an antenna group 430, an antenna group 450, and an antenna group 470, respectively. The UE1 400 receives a traffic signal from the antenna group 410 while other antenna groups 430, 450, and 470, which transmit traffic signals to other user equipment 420, 440, and 450, interfere with the UE1 400. For example, the signals transmitted from the antenna groups 430, 450, and 470 may give interference effect to the UE1 400.
In general, interference due to other antenna groups in the distributed antenna system may have two types as follows.                Inter-cell interference: Interference generated by antenna groups of other cells, and        Intra-cell interference: Interference generated in antenna groups of the same cell.        
In FIG. 4, interference generated by the antenna group 430 that is included in the same cell may be intra-cell interference with respect to the UE1 400. In addition, interference generated by the antenna groups 450 and 470 included in the nearby cell may be inter-cell interference with respect to the UE1 400. The inter-cell interference and the intra-cell interference are simultaneously received by the user terminal to thereby interrupt data channel reception thereof.
In general, the user terminal receives a wireless signal with noise and interference. For example, the reception signal may be expressed as Equation 1 below.r=s+noise+interfernce  Equation 1
In Equation 1, “r” is a reception signal, and “s” is a transmission signal. “noise” is noise that conforms to the Gaussian distribution, and “interference” is an interference signal generated in wireless communications. The interference signal may be generated in the following circumstances.                Interference by a nearby transmission location: the case in which signals transmitted from a nearby cell or a nearby antenna in the distributed antenna system interfere with a desired signal, and        Interference in the same transmission location: the case in which signals for different users interfere with each other when performing MU-MIMO transmission using a plurality of antennas in a single transmission region.        
The SNIR value may vary according to the interference, and consequently, this may influence reception performance. In general, interference is the greatest factor that hinders system performance in a cellular mobile communication system, and thus the system performance may be determined by the control of interference. The LTE/LTE-A systems have introduced various standard technologies to support coordinated multi-point transmission and reception (CoMP), which is cooperative communication, in order to control interference. In the CoMP, a network makes an overall and central control for transmission from a plurality of base stations or transmission points to thereby determine presence of interference and the intensity thereof in downlink and uplink. For example, in the case of two base stations, a central controller of a network may stop transmission of signals from the second base station in order to prevent interference with the user terminal that receives signals from the first base station.
Error correction encoding is performed in order to correct errors generated in transmitting and receiving signals in a wireless communication system. The LTE/LTE-A systems use convolution codes and turbo codes for the error correction encoding. In order to enhance decoding performance of the error correction encoding, receivers demodulate symbols modulated in quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (QAM), or 64QAM using not hard decision but soft decision. When “+1” or “−1” is transmitted from a transmission terminal, a receiver adopting the hard decision selects one of “+1” or “−1” from the reception signal and outputs the same. Contrarily, a receiver adopting the soft decision outputs information on whether “+1” or “4” is received in the reception signal together with reliability of the corresponding decision. Such reliability information may be utilized to enhance decoding performance in a decoding process.
An output value of the receiver that adopts the soft decision is generally calculated using a log likelihood ratio (LLR). When the transmission signal is applied with a binary PSK (BPSK) modulating method that is one of “+1” or “−1,” the LLR may be defined as Equation 2 below.
                    LLR        =                  log          ⁢                                    f              ⁡                              (                                                      r                    ❘                    s                                    =                                      +                    1                                                  )                                                    f              ⁡                              (                                                      r                    ❘                    s                                    =                                      -                    1                                                  )                                                                        Equation        ⁢                                  ⁢        2            
In Equation 2, “r” is a reception signal, and “s” is a transmission signal. In addition, a conditional probability density function ƒ(r|s=+1) is the probability density function of a reception signal on a condition that a transmission signal of “+1” has been transmitted. Likewise, a conditional probability density function ƒ(r|s=−1) is the probability density function of a reception signal on a condition that a transmission signal of “−1” has been transmitted. In the case of the modulating methods of QPSK, 16QAM, and 64QAM, the LLR may be expressed as an equation in a similar manner. The conditional probability density function has the Gaussian distribution in the absence of interference.
FIG. 5 illustrates a conditional probability density function according to the related art.
Referring to FIG. 5, a reference numeral 500 denotes the conditional probability density function ƒ(r|s=−1), and a reference numeral 510 denotes the conditional probability density function ƒ(r|s=+1). For example, if a reception signal has a value indicated by a reference numeral 520, the receiver may calculate the LLR as log(f2/f1) using the conditional probability density function. The conditional probability density function of FIG. 5 corresponds to the case in which noise and interference conform to the Gaussian distribution.
In mobile communication systems, such as the LTE/LTE-A systems, the base station can transmit information of more than dozens of bits to the user terminal by a one-time PDSCH transmission. At this time, the base station encodes information to be transmitted to the user terminal, and modulates the same in the manners, such as QPSK, 16QAM, 64QAM, or the like, to be thereby transmitted. Accordingly, the user terminal that has received the PDSCH creates the LLRs for dozens of encoding symbols and transfers the same to a decoder in the process of demodulating dozens of modulation symbols.
FIG. 6 illustrates a conditional probability density function on a condition that reception signals and interference signals are transmitted in a BPSK modulation method according to the related art.
Referring to FIG. 6, the noise generally conforms to the Gaussian distribution, whereas the interference may not conform to the Gaussian distribution in some cases. The main reason why the interference does not conform to the Gaussian distribution is that the interference is wireless signals for other receivers, differently from the noise. For example, in Equation 1, since “interference” is a wireless signal for another receiver, it is applied with modulation methods, such as BPSK, QPSK, 16QAM, 64QAM, or the like, to be thereby transmitted. For example, if an interference signal is modulated in “BPSK,” the interference may have probability distribution having a value of “+k” or “−k” at the same probability. The “k” above is a value that is determined by the signal intensity attenuation effect of a wireless channel.
It is assumed that the noise conforms to the Gaussian distribution in FIG. 6. It can be seen that the conditional probability density function of FIG. 6 is different from the conditional probability density function of FIG. 5.
Referring to FIG. 6, a reference numeral 620 denotes the conditional probability density function ƒ(r|s=−1), and a reference numeral 630 denotes the conditional probability density function ƒ(r|s=+1). In addition, a value 610 is determined according to signal power of the interference signal and the influence of a wireless channel. For example, if a reception signal has a value indicated by a reference numeral 600, the receiver may calculate the LLR as log(f4/f3) using the conditional probability density function. This value may be different from the LLR value of FIG. 5 because the conditional probability density function is different. For example, the LLR calculated based on a modulation method of the interference signal is different from the LLR calculated on the assumption that the interference conforms to the Gaussian distribution.
FIG. 7 illustrates a conditional probability density function on an assumption that reception signals are transmitted in a BPSK modulation method and interference signals are transmitted in a 16QAM modulation method according to the related art.
Referring to FIG. 7, the conditional probability density function may vary with the modulation method of interference. The reception signals are transmitted in the BPSK modulation method in FIGS. 6 and 7, but the interference signal in FIG. 6 is transmitted in the BPSK method, and the interference signal in FIG. 7 is transmitted in the 16QAM method. For example, even with the same modulation method of a reception signal, the conditional probability density function may vary with the modulation method of the interference signal. Consequently, the calculated LLRs may be different.
Referring to FIGS. 5, 6, and 7, the LLR may have different values depending on the assumption and calculation for the interference by the receiver. In order to optimize reception performance, the LLR should be calculated using a conditional probability density function reflecting statistical characteristics of actual interference, or the LLR should be calculated after the interference is removed in advance. For example, in the case of the interference being transmitted in the BPSK modulation method, the receiver should calculate the LLR on the assumption that the interference is transmitted in the BPSK modulation method, or after the interference modulated in the BPSK method is removed. In the case of the interference being transmitted in the BPSK modulation method, if the receiver just assumes that the interference has the Gaussian distribution or that the interference is transmitted in the 16QAM modulation method without being removed, the LLR may not be calculated to have an optimized value, so the reception performance cannot be enhanced.
Therefore, a need exists for a method and an apparatus for transferring interference-related control information in order to enhance reception performance of a user terminal that receives downlink signals in a cellular mobile communication system based on an LTE-A system.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.